ISOMORPHOUS COMPOUNDS 61 



2. Development of the Stereochemical Conceptions. 



Molecular weight measurements play a specially important 

 part in studying the cause of the isomerism between two 

 bodies of the same composition. For it is then possible 

 to decide whether there is a difference in molecular magni- 

 tude, i. e. polymerism, or a difference in construction between 

 two molecules of the same size, i.e. isomerism in the 

 narrower sense. It may be remarked here, that with 

 regard to the development of stereo-isomerism, which is 

 a special kind of isomerism attributed to the arrangement 

 of the molecule in space, molecular weight determinations 

 form an indispensable aid, since it is necessary first to 

 prove that differences between equally composed molecules 

 are in question. Hence it was 'a fortunate coincidence that 

 just at the time the theory of stereochemistry was dis- 

 covered, the new methods for the measurement of molecular 

 weights in solution, based on the theory of solutions, were 

 introduced. The interesting cases of isomerism in the 

 truxillic acids, the benzoinoximes, and many other com- 

 pounds important for stereochemistry, could hardly have 

 been so quickly and certainly brought into light without 

 the new methods for molecular weight determination. 



3. Abnormal Results for Isomorphous Compounds. 



A very striking exception appears when a substance is 

 dissolved in another isomorphous with it, as thiophene in 

 benzene. The lowering of the freezing point is then 

 decidedly less ; in the case mentioned only about two-thirds 

 of the normal amount. In such cases, as has actually been 

 shown 1 , the dissolved substance separates out in an iso- 

 morphous mixture with the solvent when the latter 



1 Van Bylert, Zeitschr. f. Pliys. CJiem. 8. 343; Beckmann, I.e. 17. 107; 

 22. 609. 



